The present invention relates generally to a soldering apparatus and soldering method using the same. More specifically, the invention relates to a soldering apparatus for soldering with a plurality of laser beams generated from laser beam oscillators employing semiconductor lasers.
There are YAG laser type and tungsten halogen lamp type soldering apparatuses put to practical use at present in soldering apparatuses employing a laser beam. The tungsten hologen lamp type soldering apparatus using a tungsten hologen lamp as a heat source transmits a laser beam having a beam spot diameter of 5 to 10 (mm). Such laser beam spot is too large for application in soldering printed boards. The YAG laser employs a laser beam as a heating beam and may be focused into a smaller spot. It will be appreciated that the YAG laser type soldering apparatus is available for soldering high density printed boards. Complete melting of solder with a YAG laser requires a large scale cooling system because only a small percentage of the electric energy is converted to beam energy. The size and cost of soldering apparatus employing a YAG laser therefore tend to be great.
Japanese Patent First Publication (tokkaisyo) No. 61-253170 exemplifies a conventional soldering apparatus employing a laser beam as a heat source. The application discloses that gas laser, semiconductor laser, solid-state laser, and excimer laser are available as a laser oscillators and the semiconductor laser has a relatively high conversion efficiency (15%) from electrical energy to beam energy. It will be noted that the semiconductor laser is very suitable for laser soldering apparatus. For these reasons, a YAG laser is generally used for a heat source. The output power of only one semiconductor laser is however too small to solder well and therefore more than one semiconductor laser is preferred. Moreover, in a high density printed circuit board within which an IC chip is disposed, lands, or portions to be soldered thereof are generally oblong and distances between the lands adjacent to each other are small. The shape of the YAG laser's beam spot is a circle. Thus, if the beam spot size of the laser beam is reduced smaller than the width of land, the laser beam is irradiated on only part of the land surface. The complete soldering of one lead wire requires displacement of a laser beam within the area to be irradiated in the longitudinal direction thereof. This results in extremely inefficient soldering. Depending on circumstances there are some cases where a plurality of spots are formed on a soldered portion. It will be appreciated that with soldering apparatus provided with only one laser beam, it is difficult to obtain fine solder connections. On the other hand, if the laser beam spot irradiated on the land is made relatively big so as to cover all surface thereof, a plurality of the lands get irradiated at the same time due to the small distances between the lead wires adjacent to each other, thereby causing a bridge of solder to be produced therebetween. It is therefore difficult for the oblong lands of a high density printed board to be efficiently soldered.